The present invention relates to a method for manufacturing a Cu alloy used for an electrode material for a vacuum circuit breaker.
As well known, a vacuum circuit breaker turns an electric current on and off by using movable and fixed electrodes disposed in a vacuum container. The material of these electrodes must provide (1) a large breaking current, (2) a small chopping current, (3) a high dielectric breakdown voltage between electrodes, (4) a difficulty in welding; and (5) only a small amount of heat during current carrying. A large number of alloys have been researched and developed for such electrode materials, and melting and casting of alloys such as Cu-Bi (bismuth) and Cu-Te (tellurium), or sintering alloys such as Cu-W (tungsten) and Cu-Mo (molybdenum) have been used practically. Currently, a Cu-Cr alloy containing 20 to 70 wt % of Cr (chromium) is commonly used as a material that has all the properties listed above. In addition, these properties required of the electrode material for vacuum circuit breakers are affected not only by the metal components but also by contained gas, such as oxygen or impurities, or the fine uniformity of the metallic structure, so that the ingredients or materials must be very pure and be melted or sintered in a protective gas such as hydrogen or argon, or in a vacuum condition.
Cr is not substantially melted into Cu at a temperature near the melting point of Cu (approx. 1,083.degree. C.). Conventional Cu-Cr alloys are made by powder metallurgy that uses Cr powders as a main material. For example, such alloys are manufactured by a sintering method that molds and sinters a mixture of Cu and Cr powders, or a melting-infiltrating method wherein a mixture of Cr powders and a small amount of Cu powders are molded and sintered to obtain a porous body, to which molten Cu is impregnated. In this case, the Cu-Cr alloy manufactured by using these methods includes Cr particles dispersed in the Cu base, but most of the dispersed Cr particles are almost as large as the ingredient powders. And only a small amount of fine Cr particles is contained in the alloy, which is formed such that Cr melts into Cu during heating and precipitates into Cu during cooling.
The conventional Cu-Cr alloy manufacturing method uses as materials Cr powders, which are formed such that Cr masses produced by Alumit process or electrolytic method are ground mechanically. As well known, Cr is easily oxidized, so that the surfaces of the Cr powders are covered with strong oxide films during grinding. In addition, the Cr powders are mixed with Cu powders by using a ball mill or a V mixer, and the Cr powders are oxidized even during this operation. The oxide film is thermally stable and can not be decomposed or reduced at a normal sintering temperature. Thus, the Cu-Cr alloy obtained by the powder metallurgy disadvantageously contains a large amount of oxygen. In the sintering method, the oxide film hampers the fusion of Cu and Cr, while in the melting-infiltrating method, it prevents Cu particles from infiltrating into the porous body, causing defects such as voids in the structure. These defects may reduce the breaking current or dielectric breakdown voltage.
Furthermore, in the conventional Cu-Cr alloy manufacturing method, the size of the Cr particles is determined by the size of ingredient powders. The reduction of the size of the Cr powders, however, is limited due to manufacturing techniques, and the fine Cr powders have increased surface areas, resulting in the correspondingly increased amount of oxygen contained therein. Thus, the conventional Cu-Cr alloy is unlikely to have fine Cr powders in a Cu base and its average particle size is limited to approx. 150 .mu.m. The size of the Cr particles particularly affects the chopping current, which disadvantageously increases with the increasing of the size of the dispersed Cr particles. The uniformity of the dispersion of the Cr powders also affects the chopping current, and the value of the chopping current fluctuates when the dispersion is not uniform. If, however, the time required for mixture by using a ball mill is extended in order to ensure uniform dispersion, the oxidization of the ingredient powders is facilitated correspondingly.
As a method that solves the problems related to the sintering or the melting-infiltrating methods, Japanese Patent Application Laid Open No. 4-71970 discloses a method that uses an arc or laser for melting.
This method mixes, for example, Cr and Cu powders together, compresses, molds, and sinters the mixture to manufacture a columnar block; uses this block as an arc electrode to melt it gradually from one end by using an arc heat, and then sequentially solidifies it in a water-cooled mold. Other than the arc, the use of a laser or high-frequency plasma has been disclosed. This method can provide an alloy with uniformly dispersed fine Cr particles. Due to the use of the Cr powders, however, this method fails to satisfy the need to reduce the content of oxygen. Since this is a sequential melting and solidifying method that gradually melts the block from one end, the Cr and Cu powders must be as fine as possible and be uniformly mixed throughout the block in order to obtain a Cr-Cu alloy containing predetermined components throughout the casting mass. Thus, this method can not avoid the use of the powder materials and a mixing process that may increase the amount of oxygen.
In addition, the Cr-Cu alloy may contain Te, Bi, Sb, or Zn to improve resistance to the welding or to reduce the chopping current. Since these elements have a high vapor pressure, the temperature during melting must not be unnecessarily increased in order to avoid evaporation losses. Even if the alloy consists of only Cr and Cu, it is not preferable to unnecessarily increase the melting temperature, as evaporated Cu or Cr contaminates a melting furnace. Melting with an arc or laser necessarily increases the temperature up to several thousand degrees (Celsius), so that the temperature can not be controlled easily.
It is an object of this invention to manufacture a Cu-Cr alloy electrode material for a vacuum circuit breaker that has a low oxygen content and few defects in the metallographic structure, and in which fine Cr particles are uniformly dispersed in a Cu base.